Think about it, Nvidia is going down the tubes. They would probably do anything to increase their profits. What better way than to pay Valve a large sum of money and tell them to optimize the game for Nvidia cards, or maybe their upcoming card(s). If they make their video card directly compatible with HL2, and have Valve back them up, their sales would go through the roof. Of course, this may very well not be true. I think Nvidia did something like this with Doom 3, not completely sure though.